Translocation of tRNAs across mitochondrial membranes is a receptor-mediated active transport process dependent on the pmf (Bhattacharyya and Adhya, 2004). A large tRNA import complex from the inner membrane of Leishmania mitochondria catalyzes translocation into mitochondria and phospholipid vesicles. When tRNA binds to its receptor, activation of the complex leads to opening of the import channel (Goswami et al., 2006). Two tRNA-binding proteins that are parts of the channel have been isolated (Bhattacharyya and Adhya, 2004). These proteins are described by Chatterjee et al. (2006) and Goswami and Adhya (2006). Mahata et al. (2006) showed that human mitochondria also take up cytosolic tRNAs.

The RNA import complex (RIC) from the mitochondrion of the kinetoplastid protozoan Leishmania tropica contains two subunits that directly bind to import signals on two distinct subsets of tRNA and interact with each other allosterically. A third subunit, RIC9, has intrinsic affinity for both types of tRNA and is essential for import in vivo. Antibody against RIC9 inhibits the import of both types of tRNA into mitoplasts in vitro, but failed to inhibit the binding of these tRNAs to their respective receptors, indicating that RIC9 acts in a subsequent step. tRNA is transferred from its cognate receptor to RIC9, followed by translocation across the membrane and release as free tRNA in the inner compartment. Transfer required ATP. tRNA movements were sensitive to uncouplers and inhibitors, suggesting distinct roles of the electrical and chemical components of the proton motive force generated by vectorial proton translocation accompanying ATP hydrolysis. RIC9 was shown to make stable contacts with RIC8A, a tRNA receptor, and RIC6, a membrane-embedded component. The results have implications for the mehanism of tRNA import.